How to move 4 objects symmetrically outward/inward on the same origin?
Take one of the cubes symmetrically placed on the plane and mirror it on the x and y axis.  Go to the mirror modifier and set the mirror object as the plane (which they will be placed on at the four corners).  Now if you go into edit mode you can select the one cube and move it to one corner, and the mirrored cubes will move symmetrically to the other corners.  Good luck!